refactor(arch): introduce IEbookRepository, ISyncBroadcaster, fix EpubReader path resolution

Critical fixes (review findings #1, #2, #3):
- Create IEbookRepository abstraction in Application layer
- Remove illegal EF Core dependency from IngestEbookCommandHandler
- Create EbookRepository implementation in Infrastructure/Persistence
- Create ISyncBroadcaster in Application/Abstractions/Messaging
- Create SignalRSyncBroadcaster in Infrastructure/RealTime
- Move UpdateReadingProgressCommandHandler from Infrastructure → Application
- Add EbookId to GetReaderPageQuery and IEpubReader signature
- Rewrite EpubReaderService: DB-resolved file path, remove auto-provisioning
- Split EpubService.cs into EpubReaderService.cs + EpubMetadataExtractor.cs
- Add CurrentEbookId to IReaderNavigationService and ReaderNavigationService
- Update WasmEpubReader and /api/epub endpoint for new signature

High severity fixes (#4, #6, #7, #8, #16):
- Change BookStorageService registration from Singleton → Scoped
- Fix empty catch{} in ReaderCanvas JS interop init — now logs warnings
- Replace all Console.WriteLine with ILogger in KnowledgeService + ReaderCanvas
- Cache JsonSerializerOptions as static field in KnowledgeService
- Wrap SyncService Task.Run body in comprehensive try/catch with ILogger

Medium/Low fixes (#11, #13, #14, #15, #18, #20):
- BookIngestionModal.DisposeAsync now nullifies _epubBytes (50MB array)
- KnowledgeCoordinator.OnGraphUpdated: Action<T> → Func<T, Task>
- BookStorageService: Path.Combine → forward-slash string interpolation
- SignalR CancellationToken passed as named parameter (not payload arg)

src/NexusReader.Web/Program.cs

@@ -239,10 +239,10 @@ app.MapStaticAssets();
 app.MapHub<NexusReader.Infrastructure.RealTime.SyncHub>("/synchub");
 
 // API endpoint for WASM client to fetch EPUB content
-app.MapGet("/api/epub/{index}", async (int index, IEpubReader epubService, ClaimsPrincipal user) =>
+app.MapGet("/api/epub/{ebookId:guid}/{index:int}", async (Guid ebookId, int index, IEpubReader epubService, ClaimsPrincipal user) =>
 {
     var userId = user.FindFirstValue(ClaimTypes.NameIdentifier);
-    var result = await epubService.GetEpubContentAsync(index, userId);
+    var result = await epubService.GetEpubContentAsync(ebookId, index, userId);
     
     if (result.IsSuccess) return Results.Ok(result.Value);